Prior to the present invention, as shown by Webb, U.S. Pat. No. 4,116,980 and Markezich et al, U.S. Pat. No. 4,128,574, assigned to the same assignee as the present invention, aromatic bis(ether anhydride)s of the formula, ##STR1## where R is a C.sub.(6-30) divalent aromatic organic radical, and the corresponding aromatic bis(ether dicarboxylic acid)s were made by effecting reaction between the appropriate aromatic bis(ether-imide) and aromatic anhydride in the presence of water and an imide-anhydride exchange catalyst. Imide-anhydride exchange was achieved by heating the imide-anhydride exchange reaction mixture at elevated temperatures in the presence of water.
An improvement was effected by Webb when the aforementioned mixture of aromatic bis(ether imide) and phthalic anhydride was heated under pressure in the presence of water and an N-organo aromatic imide was selectively removed from the liquid phase mixture by venting the vapor phase therefrom. Although the aforementioned procedure of heating the imide-anhydride exchange mixture under sealed conditions provides valuable exchange results, the procedure requires several hours of heating and venting before desirable yields of aromatic bis(ether anhydride) reaction product can be recovered from the exchange reaction mixture.
Efforts to improve the rate at which equilibrium was achieved in an imide-anhydride exchange reaction utilizing the aromatic bis(ether-imide) in molten form and contacting it with phthalic acid and water, often resulted in poor mixing of the aqueous feed stream with the molten aromatic bis(ether imide).
The present invention is based on the discovery that an aqueous phthalic acid exchange-catalyst solution having a phthalic anhydride concentration (25-35% by weight) or sufficient to provide a feed viscosity of at least 25 to 90 centipoises at 25.degree. C., has been found to provide optimum mixing characteristics with molten aromatic bis(ether imide), of the formula, ##STR2## and significant improvements in the rate of production of aromatic bis(ether anhydride) of formula (1), where R.sup.1 is a monovalent organo radical selected from the class consisting of C.sub.(1-8) alkyl radicals, and C.sub.(6-13) aromatic radicals.